Fouling is generally defined as the accumulation of unwanted materials on the surfaces of processing equipment and in petroleum processing, is the accumulation of unwanted deposits from a fluid of hydrocarbon origin on heat transfer surfaces in process units. By “heat transfer surfaces” is meant a surface across which heat is transferred from or to—usually, to—the hydrocarbon fluid, for example, the tube surfaces in furnaces and heat exchangers. Fouling has been recognized as a nearly universal problem in the design and operation of such equipment and affects the operation of equipment in two ways. First, the fouling layer has a low thermal conductivity. This increases the resistance to heat transfer and reduces the effectiveness of the unit. Second, as deposition occurs, the cross-sectional area is reduced, which causes an increase in pressure drop across the apparatus and creates inefficient pressure and flow in the unit.
Fouling in heat transfer equipment used for streams of petroleum origin can result from a number of mechanisms including chemical reactions, corrosion and the deposit of materials made insoluble by the temperature difference between the fluid and heat exchange wall. When crude oils are passed through heat transfer equipment, for example, when the heating medium on the far side of the exchanger is much hotter than the oil, relatively high surface or skin temperatures can result and asphaltenes in the crude can precipitate from the oil and adhere to these hot surfaces. The presence of insoluble contaminants may exacerbate the problem: blends of a low-sulfur, low asphaltene (LSLA) crude oil and a high-sulfur, high asphaltene (HSHA) crude, for example, may be subject to a significant increase in fouling in the presence of iron oxide (rust) particulates. Subsequent exposure of the precipitated asphaltenes over time to the high temperatures then causes formation of coke as a result of thermal degradation.
Another common cause of fouling can result from the presence of salts and particulate which precipitate from the crude and adhere to the heated surfaces. Inorganic contaminants can play both an initiating and promoting role in the fouling of whole crude oils and blends: iron oxide, calcium carbonate, silica, sodium and calcium chlorides have all been found to be attached directly to the surface of fouled heater tubes and throughout coke deposits on the heater surfaces. Desalter units are still the only opportunity refineries have to remove such contaminants and inefficiencies often result from the carryover of such materials with the crude oil feeds.
Equipment fouling is costly to petroleum refineries and other plants in terms of lost efficiencies, lost throughput, and additional energy consumption, and, with the increased cost of energy, heat exchanger fouling has a greater impact on process profitability. Higher operating costs also accrue from the cleaning required to remove fouling. While many types of refinery equipment are affected by fouling, cost estimates have shown that the majority of profit losses occur due to the fouling of whole crude oils, blends and fractions in pre-heat train exchangers.
The cleaning process, whether chemical or mechanical, in petroleum refineries and petrochemical plants often causes costly shutdowns; most refineries practice off-line cleaning of heat exchanger tube bundles based on scheduled time or usage or on actual monitored fouling conditions. Reduction in the extent of fouling will lead to increased run lengths, improved performance and energy efficiency while also reducing the need for costly fouling mitigation options.
It would be desirable to prevent the precipitation/adherence of particulates and asphaltenes on heated transfer surfaces before the particulates can promote fouling and the asphaltenes become thermally degraded or coked. By keeping asphaltenes in solution and particulates in suspension, the initial precipitation and subsequent thermal degradation of organic deposits and accumulation of particulates can be substantially reduced.
One contributing cause of fouling is the processing of blends of petroleum oils of different origin in the refinery. Blending of oils in refineries is common, but certain blends are incompatible and cause precipitation of asphaltenes that can rapidly foul process equipment. Although most blends of unprocessed crude oils are not potentially incompatible, once an incompatible blend is obtained, the rapid fouling and coking that results usually requires shutting down the refining process in a short time. One mitigating approach has been to ensure that two or more potentially incompatible petroleum oils are blended in a manner which maintains compatibility. U.S. Pat. No. 5,871,634 (Wiehe) discloses a method of blending that includes determining the insolubility number (In) for each feedstream and determining the solubility blending number (SBN) for each stream and combining the feedstreams such that the SBN of the mixture is greater than the In of any component of the mix. In another method, U.S. Pat. No. 5,997,723 (Wiehe) uses a blending method in which petroleum oils are combined in certain proportions in order to keep the SBN of the mixture higher than 1.4 times the In of any oil in the mixture. Reference is made to U.S. Pat. Nos. 5,871,634 and 5,997,723 for a description of the methods by which SBN and In may be determined. Some blending guidelines suggest a SBN/In blend ratio>1.3 and a Δ(SBN−In)>10 to minimize asphaltene precipitation and fouling. However, these blends are designed for use as a passive approach to minimizing asphaltene precipitation.
In related application Ser. No. 11/506,901, a method is described for reducing asphaltene induced fouling and particulate induced fouling by blending crude oils with certain high solvency dispersive power (HSDP) crude oils. While this method is effective as described, it may not be convenient for each and every refinery to make use of the method since access to cargoes of the proper HSDP crudes may not be easy.